Diagnostic ultrasound ( PDFDrive )

10 PART I Physics
FIG. 1.13 B-Mode Imaging. A two-dimensional (2-D), real-time image is built by ultrasound pulses sent down a series of successive scan lines.
Each scan line adds to the image, building a 2-D representation of echoes from the object being scanned. In real-time imaging, an entire image is
created 15 to 60 times per second.
of difering amplitude. To generate a 2-D image, multiple ultrasound
pulses are sent down a series of successive scan lines (Fig.
1.13), building a 2-D representation of echoes arising from the
object being scanned. When an ultrasound image is displayed
on a black background, signals of greatest intensity appear as
white; absence of signal is shown as black; and signals of intermediate
intensity appear as shades of gray. If the ultrasound
beam is moved with respect to the object being examined and
the position of the relected signal is stored, the brightest portions
of the resulting 2-D image indicate structures relecting more
of the transmitted sound energy back to the transducer.
In most modern instruments, digital memory is used to store
values that correspond to the echo intensities originating from
corresponding positions in the patient. At least 2 8 , or 256, shades
of gray are possible for each pixel, in accord with the amplitude
of the echo being represented. he image stored in memory in
this manner can then be sent to a monitor for display.
Because B-mode display relates the strength of a backscattered
signal to a brightness level on the display device, it is important
that the operator understand how the amplitude information in
the ultrasound signal is translated into a brightness scale in the
image display. Each ultrasound manufacturer ofers several options
for the way the dynamic range of the target is compressed for
display, as well as the transfer function that assigns a given signal
amplitude to a shade of gray. Although these technical details
vary among machines, the way the operator uses them may
greatly afect the clinical value of the inal image. In general, it
is desirable to display as wide a dynamic range as possible, to
identify subtle diferences in tissue echogenicity (see Fig. 1.11).
Real-time ultrasound produces the impression of motion by
generating a series of individual 2-D images at rates of 15 to 60
frames per second. Real-time, 2-D, B-mode ultrasound is the
major method for ultrasound imaging throughout the body and
is the most common form of B-mode display. Real-time ultrasound
permits assessment of both anatomy and motion. When images
are acquired and displayed at rates of several times per second,
the efect is dynamic, and because the image relects the state
and motion of the organ at the time it is examined, the information
is regarded as being shown in real time. In cardiac applications
the terms 2-D echocardiography and 2-D echo are used to describe
real-time, B-mode imaging; in most other applications the term
real-time ultrasound is used.
Transducers used for real-time imaging may be classiied by
the method used to steer the beam in rapidly generating each